As is known, electric motors which are controlled by way of a supply frequency are increasingly used in all kinds of textile machinery but are still under discussion for ring spinning machines.
Each individual spinning position of a ring spinning machine comprises three basic operative elements which must be moved, viz. a spindle, a drawframe and a ring carrier or ring bank. An individual spindle is usually associated with the spinning position but the drawframe and the ring bank extend over a number of spinning positions, as a rule, over the whole length of one side of the machine. For the reasons given hereinafter, endeavors have been made to "decentralize" the conventional central drive system of the ring spinning machine which has been in the form of a main driving motor having transmissions to distribute the driving power to the various operative elements.
The main reasons for these endeavors with respect to a single spindle drive are higher productivity, energy saving, noise reduction, higher speeds, and fewer yarn breakages.
For an individual drafting arrangement drive, the reasons are no change gears, simple and rapid control, the possibility of remote control and the possibility of fine adjustment.
When the three operative elements hereinbefore mentioned are considered individually, the endeavors have met with considerable success. A number of "individual drive systems" are available which drive the spindles individually (or in groups) and give the drafting arrangement its own independent drive. The ring bank can be moved either together with the drafting arrangement or by an independent drive. However, despite many such proposals, no individual drive system has yet been introduced in practice. Although the various decentralized drive systems for the spindles, drafting arrangement and ring bank can still be further improved or optimized, the main outstanding problems are the co-operation of these drives with one another and within the drive system of the drawframe, particularly for starting and stopping the machine. Deviations from the programmed speeds are very likely to occur in this phase and endanger the technological parameters of the spun yarn. Making a drive suitable for practical use is therefore a very difficult job. More particularly, the spindles have to be accelerated from a standstill to their operating speed (or brought to a standstill) with a programmed starting slope if yarn breakages are to be avoided. Further, the drafting arrangement (and ring bank) must so move relative to the spindles that no yarn breakages occur and the yarn quality produced during starting (and stopping) corresponds (is as near as possible the same as) the yarn quality produced in normal operation.
A drive system meeting these requirements must also be economic to produce if it is to be able to compete with conventional central drives.
It has long been known, for example, from DOS 2 203 833, that driving motors speed-controlled by way of the supply frequency offer possibilities of solving the problems mentioned. However, drive systems devised for such motors cannot readily meet all requirements. As soon as (or as long as) such a motor runs in synchronism with its supply frequency, the motor can be maintained (within its load limits) in a desired relationship to other such motors. However, a distinctive feature of such motors is that, if the motor is designed with a rational load bearing capacity, the motor either does not start immediately from a standstill (or decelerate to a standstill) in synchronism with the supply frequency and is instead uncontrollable below a critical speed (minimum speed or minimum frequency), and/or the motor cannot produce an adequate and exactly maintained acceleration torque from a standstill. This feature causes problems, particularly in connection with the driving of drafting arrangements, as will be described in greater detail hereinafter.
The drafting arrangement of a ring machine comprises a number of units consisting of cylinder/roller pairs. The inter-unit speed ratios determine compliance with the yarn count while the speed ratio between the front roller unit and the yarn-twisting spindle is decisive for the level of twist in the yarn. The units must start from a standstill and, a stoppage, return to a standstill "with gearwheel accuracy"--i.e., in a predetermined relationship of the angles of rotation. Also, the drafting arrangement requires a minimum starting acceleration because, at stoppage of the machine, the yarns preferably remain connected to the spindles and the spindles restart so rapidly that the yarns are tensioned and form a balloon. If the drafting arrangement cannot accelerate to its working speed fast enough in these conditions, mis-twisting and eventually massive yarn breakages occur. Also, for the same reasons, the rotations of the drafting cylinders and rollers should, at stoppage of the machine, be maintained until stoppage (or until a low speed) of the spindles. But, because of considerable differences between the moment of inertia of the spindles and that of the drafting arrangement, this requirement causes considerable problems.
Preferably, an "individual drafting arrangement drive system" comprises at least one drive for the front roller unit and one drive for the other drafting units and possibly even one drive per unit. The reasons hereinbefore set out make it impossible to embody such drive systems using low-cost motors speed-controlled by way of their supply frequency, without taking further action, to maintain the necessary relationships below a critical speed.
German O.S. 2 849 576 describes a drive system for a ring spinning machine or machines which includes two motors for each drafting unit. In addition, each motor is adapted to be coupled by way of a clutch and a belt connection to the corresponding drawframe unit. The control for these clutches is arranged to produce effects in the yarn; however, the control is not related to synchronization of the motors.